Blind signature specification

Blind signatures

WIP

Blind signatures use the following primitives which need to be saved into a file system or transferred over the network:

BigNum (32B): since the curve is secp256k1, the bignums are 256bits long (32 bytes). They are serialized using OpenSSL's BN_bn2binpad

Eliptic curve points (33B): again, since we're using secp256k1, the coordinates X and Y are 32 bytes. We use compressed format compatible with OpenSSL's EC_POINT_set_compressed_coordinates. I tried to replicate how it's stored in other places and how OpenSSL behaves, so the data should be loadable by other software just like a bitcoin private key

• the first byte is 0x02 or 0x03 (which determines which of the two Y coordinates it uses, the difference is that one is even and one is odd. Other bits are ignored by Bitmessage
• the next 32 bytes are the X coordinate (which is a bignum)

Private key (32B): private key is a 256 bit bignum

Public key (35B): public key is two bytes of parameters followed by an EC Point as described above

• first byte is expiration, split into two 4-bit parts. The lower 4 bits are month (0 - 11), and the higher 4 bits are year (add 2020 to get the actual year). For example 0x03 is April 2020.
• second byte is value (1-255)
• the rest of the bytes (33B) is an EC point

Random integer provided by the signer to the requester during signing (33B): is an EC point

Blinded signature(32B): Blinded signature is a 256bit bignum

Unblinded Signature (65B): is a bignum (32B) followed by an EC point (33B)

Signature chain (100 bytes per level):

• Begins with the CA pubkey (35 bytes)
• on the next levels the intermediary pubkey (35B) and a signature of that pubkey signed by the parent pubkey (65B)
• and on the last level, the signature of the message signed by the parent pubkey (65B)